Covered stents, or stent grafts, have been used to treat a variety of medical conditions, including aneurysms, occluded vessels, and restenosis. Various materials and methods have been used to create coverings, or grafts, that may be applied to stents to form stent grafts. Typically, stent grafts are designed to be substantially nonporous. These nonporous stent grafts may be designed to prevent the flow of blood through the graft material (such as a stent graft for excluding an aneurysm) or to limit or control cellular in-growth to prevent restenosis. In fact, stent grafts produced using porous graft materials often may be “pre-clotted” to reduce the permeability of the graft material prior to implanting the stent graft within a patient's body.
Some stent grafts have been designed to have some degree of porosity. These grafts may be manufactured to have a specific pattern or configuration designed to achieve a desired result such as promoting or diminishing endothelial growth or controlling the permeability of fluids through the grafts. The patterns may be generated by processes such as weaving, extrusion, laser marking, and mechanical punching. Typically, additional manufacturing steps are required to generate a pattern on a graft after the graft itself has been produced.
Electrospinning is a process for creating a non-woven network of fibers using an electrically charged solution that is driven from a source to a target with an electrical field. More specifically, a solution is driven from an orifice, such as a needle. A voltage is applied to the orifice resulting in a charged solution jet or stream from the orifice to the target. The jet forms a conical shape, termed a Taylor cone, as it travels from the orifice. As the distance from the orifice increases, the cone becomes stretched until the jet splits or splays into many fibers prior to reaching the target. The fibers are extremely thin, typically in the nanometer range. The collection of fibers on the target forms a thin mesh layer of fibrous material.
It may be desirable to provide a patterned graft material in a single manufacturing step. It also may desirable to use an electrospinning process to form a patterned graft material having a precisely controlled porosity.